原数据 
              'A' ,'1'
              'A', '2'
              'B', '1'
              'B', '2'
         实现后
              'a','1->2'
              'B','1->2'
   

select col1,
col2=stuff((select '->'+ltrim(col2) from tb where col1=t.col1 for xml path('')),1,2,'')
from tb t
group by col1